Electrochemical fuel cells convert reactants, namely fuel and oxidant streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
During operation of a solid polymer electrolyte fuel cell, a fuel stream is supplied to the anode of the fuel cell and is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Solid polymer fuel cells typically employ a membrane electrode assembly (“MEA”) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The membrane is an ion conductive (typically proton conductive) material and acts as a barrier for isolating the fuel and oxidant streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the fuel and oxidant streams. The separator plates act as current collectors and provide support for the MEA.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. A fuel cell stack typically includes fuel inlet(s) and fuel supply manifold(s) for directing the fuel to the plurality of anodes and oxidant inlet(s) and oxidant supply manifold(s) for directing the oxidant to the plurality of cathodes. The fuel passage extends from the fuel inlet(s) to the fuel outlet(s) and transverses the anodes. The stack often also includes a coolant inlet and coolant supply manifold for directing a coolant fluid to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes fuel exhaust manifold(s) and fuel outlet(s) for expelling the unreacted fuel gases, and oxidant exhaust manifold(s) and oxidant outlet(s) for expelling the unreacted oxidant and reaction products as well as a coolant exhaust manifold and coolant outlet for the coolant stream exiting the stack. The stack manifolds, for example, can be internal manifolds, which extend through aligned openings formed in the separator layers and MEAs, or can comprise external or edge manifolds, attached to the edges of the separator layers.
Conventional fuel cell stacks are sealed to prevent leaks of reactants to the surrounding environment and to prevent inter-mixing of the fuel and oxidant streams. Fuel cell stacks typically employ fluid tight resilient seals, such as elastomeric gaskets between the separator plates and membranes. Such seals typically circumscribe the manifolds and the electrochemically active area of the MEA. Sealing is affected by applying a compressive force to the resilient gasket seals.
The operation of a hydrogen fuel cell system creates a concern about fuel leakage from the fuel cell stacks. Hydrogen is a combustible fuel, and from a safety perspective, it is desirable to control or reduce the leakage of hydrogen to the surrounding environment. Various measures can be taken to reduce the risk of loss of fuel to the exterior of the fuel cell, including visual inspection of the fuel cell system, and gas leak detectors and cell voltage monitoring to determine if fuel is lost from the system. Each of these approaches facilitates the detection of a gas leak, but does not prevent the release of hydrogen to the surrounding environment if a leak occurs. Additional steps are usually needed and taken to stop the leak once it is detected. Furthermore, the fuel that leaks from the system is difficult if not impossible to recover.
Other techniques have been contemplated to watch for leakage of fuel across the membrane. For example, oxidant temperature sensors have been suggested for use in the oxidant exhaust to shut down the fuel cell and sound an alarm in the event of an abnormal rise in temperature, which could occur from combustion within the fuel cell.
The fuel stream supplied to the anode typically comprises hydrogen or is reformed to convert at least a portion to hydrogen. For example, the fuel stream can be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol can be employed. The oxidant stream, which is supplied to the cathode separator plate, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Fuel outlets and oxidant outlets are not invariably necessary, or need not invariably be open. U.S. Pat. No. 5,366,818 describes fuel cell designs and operating conditions that permit the removal of accumulated water within the fuel stream exiting the fuel passage through a fuel outlet port. Water removal through the fuel outlet port permits the operation of a fuel cell in a “dead-ended” mode on the oxidant side that is, with a closed oxidant outlet port. That is, the oxygen-containing oxidant stream can be fed to the cathode and consumed substantially completely, producing essentially no outlet stream from the cathode.
U.S. Pat. No. 6,106,964 discloses that a solid polymer fuel cell typically has both a fuel exhaust stream and an oxidant exhaust stream exiting the fuel cell via fuel and oxidant exhaust ports, however one of the reactant passages can be essentially dead-ended with optional intermittent venting of inert components.
A fuel cell system, as used herein, refers to a fuel cell or a fuel cell stack and various external apparatus, equipment or components associated with the fuel cell or fuel cell stack. Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits.
The performance of the fuel cell or fuel cell stack typically increases with increasing pressure of the reactants. As reactant pressure increases, the power density of the fuel cell increases. Most fuel cell stacks are supplied with fuel streams and oxidant streams at a relatively high pressure, for example greater than atmospheric pressure. The pressure is selected to provide the desired fuel cell performance, which generally is the maximum power output that can be obtained from the fuel cell stack. Previously, it has been supposed that proton exchange membrane fuel cells should operate on fuel streams supplied at pressures of 10 psig to 65 psig, although higher and lower pressures may have also been contemplated.
It is therefore advantageous to reduce or eliminate leakage of fuel from a fuel cell to the surrounding environment. In particular, it is desirable from a safety perspective to provide a method for operating a fuel cell that reduces the leakage of hydrogen from the fuel cell, rather than detecting the hydrogen that leaks during operation and thereafter reacting to the detected leak.